Soliton transmission systems can potentially provide exceedingly high information transmission capacity over long distances. In ultra-long distance systems such as transcontinental and transoceanic systems, optical amplifiers periodically boost the power of propagating information-bearing soliton pulses to compensate for losses experienced in the fiber transmission medium. Unfortunately for ultra-long distance systems, however, the maximum information bit rate for a single channel is set by the amount of jitter in pulse arrival times generated by two different effects. One is the Gordon-Haus effect and the other is an acoustic interaction effect.
The Gordon-Haus effect is caused by the interaction of soliton pulses with amplifier spontaneous emission noise present along the transmission medium. J. P. Gordon et al. describe this effect in Optic Letters, Vol. 11, No. 10, pp. 665-7 (1986). Amplifier spontaneous emission noise randomly alters both the amplitude and carrier or channel frequency of the soliton pulses, resulting in a jitter in pulse arrival times. Pulse jitter can cause a soliton pulse to shift into the time interval reserved for a neighboring soliton pulse. The result, often known as intersymbol interference, is an error in the received information.
The acoustic interaction effect results from electrostriction caused by the electromagnetic field of the soliton pulse. Such electrostriction creates an acoustic wave which crosses the fiber core, changing the index of refraction of the region of the optical fiber proximate the pulse. After the pulse passes, the index of refraction of that region of the optical fiber returns to its normal state. If the soliton pulses are being transmitted at a sufficiently high rate (1 Gbit/sec or greater), a subsequent soliton pulse may arrive in that region before the effect of the prior pulse has dissipated. This can cause a shift in the frequency peak of that soliton pulse.
It has been found that the Gordon-Haus effect and the acoustic interaction effect can be reduced by the use of filters whose nominal filter center frequencies are different from the center frequencies of adjacent filters. The center frequency of each successive optical filter can be translated up or down in a predetermined pattern such as frequency increasing, frequency decreasing, or a combination of both. Use of such filters, referred to as "sliding frequency guiding filters" by their inventors and described in U.S. Pat. No. 5,357,364, incorporated by reference herein, creates a transmission environment which is substantially opaque to noise while remaining perfectly transparent to solitons. In this transmission system, soliton pulses are launched at a particular frequency and, as they propagate along the transmission medium, are accelerated toward successively different frequencies determined by the optical filter center frequency for each of the sliding-frequency guiding filters. A nonlinear interaction of the soliton pulse with the optical fiber causes each soliton pulse to generate new frequency components to match the frequency of each filter, causing each soliton pulse to emerge at a substantially different frequency from the launch frequency. The frequency shift overcomes deviations caused by the Gordon-Haus and acoustic interaction effects.
Since the generation of new frequency components by the soliton pulse is the result of a nonlinear effect, linear pulses, such as noise, cannot generate the new frequencies required to follow the changing frequencies of the sliding filters. Such pulses, therefore, eventually suffer catastrophic energy loss from the action of the sliding filters as they are left behind in a different frequency band. Stronger filter response characteristics are therefore possible, enabling greater jitter reduction than prior art systems without incurring the usual penalty of exponentially rising noise from the excess amplifier gain required to overcome the additional filter loss. The reduction of the Gordon-Haus and acoustic interaction effects dramatically improves transmission of solitons over transoceanic distances.
In a given optical transmission system supporting soliton propagation, there is a maximum and minimum soliton pulse energy which can be effectively propagated. Soliton pulses with energies above the maximum can generate additional soliton pulses. Soliton pulses with energies below the minimum will dissipate. In both cases, errors will be introduced into the transmission. Optical transmission systems preferably provide a reasonable tolerance for transmission energies.